It is well known that the lithographic offset printing plate is treated chemically so that there are printing and non-printing areas so that the printing area is receptive to ink. The non-printing area, on the other hand, is hydrophilic and accepts moistening fluid. In order to achieve the desired printing, a film of moistening fluid is applied to the surface of the plate which is retained by the hydrophilic area but which beads up on the printing area thereby allowing the printing area to receive the printing ink. The non-printing area, thus, is separated and isolated from the inking rollers by the film of moistening fluid. In this manner only the printing or image area receives ink which is then transferred to the blanket cylinder and onto the paper on which the image is printed. The purpose of lithographic dampening systems is to feed moistening or dampening fluid to the printing plate.
One method of categorizing dampening systems is in terms of whether or not a return path for fountain solution exists between the plate cylinder on the printing press and the metering elements of the dampening system. Dampening systems of the brush and spray types contain a physical gap between the means for metering the dampening fluid, or fountain solution, and the plate cylinder and, as a result, fountain solution can travel in one direction only, that is toward the plate. Thus, dampening systems in this category are referred to as the non-contact or one-way type.
In contrast, dampening systems of the contact type do not contain a physical gap in the path of the fountain solution, thus making is possible for fountain solution to travel to and fro between the metering means in the dampening system and the plate cylinder. Thus, this second general class of dampening systems has also been referred to as the two-way or contact type dampening system. Contact or two-way type dampening systems are further divided in continuous types and conventional or ductor types. The vase majority of modern contact type dampeners are of the continuous type and generally contain either three or four rollers; hence this class of dampeners are often referred to as either a three-roller or a four-roller design.
FIG. 1 illustrates a typical modern non-contact type dampener which uses a rotary brush to flick off fine droplets of fountain solution from a pan roller in a pan and to propel the droplets across a gap toward a chrome surfaced vibrating roller in the dampening system. Metering of the amount of dampening fluid that is delivered to the vibrating roller is accomplished by varying the speed at which the pan roller delivers fountain solution to the point where it is flicked away by the bristles of the rotating brush. A major advantage of this type of dampener is that no ink is fed back to contaminate the metering elements and the fountain solution supply system by virtue of the physical gap which exists in the path from the pan to the plate cylinder. There are, however, two major drawbacks to this type of dampening system design. First, there is no easy way to control the rate of flow in the lateral direction along the vibrating roller. Second, and perhaps more importantly, when overfeeding of dampening fluid to the plate occurs i.e. an excess of dampening fluid, the system cannot automatically compensate by returning some of the excess dampening fluid back to the supply system. This can cause a degradation in print quality and can result in more waste. As a result, brush systems require more operator time and skill to achieve good quality printing.
Spray type dampeners meter dampening solution by the use of plurality of atomizing spray nozzles which direct a pulsed flow of fine drops of fluid across a physical gap onto a dampening or inking roller. The use of many nozzles makes it possible to control flow laterally and the one-way character of the design eliminates the problem of ink contamination in the dampening fluid supply system. Spray systems, however, retain the drawback of one-way designs in that there is no automatic compensation for overfeeding. Thus print quality often suffers when using spray type dampeners. A typical example of a spray dampener is illustrated in U.S. Pat. No. 4,469,024 to Schwartz et al. issued Sep. 4, 1984.
U.S. Pat. No. 4,724,764 to MacPhee et al issued Feb. 18, 1988 illustrates various embodiments of the three-roller continuous contact type dampening system. U.S. Pat. No. 4,777,877 to Lemaster issued Oct. 18, 1988, illustrates one embodiment of the four-roller continuous contact type dampening system. In both three and four-roller designs metering of the dampening fluid is accomplished by a pair of rollers squeezed together so as to limit the amount of fluid which passes through their junction or nip. More specifically, the amount of fluid metered is adjusted by varying either the speed, pressure setting, or hardness of the rollers. This scheme requires that an excess amount of fountain solution be fed to the inlet side of this metering nip. This is normally accomplished by partially immersing one of the rollers in a pan containing fountain solution, so that an excess of dampening fluid is carried by the roller from the pan to the metering nip, with excess fluid automatically flowing back down into the pan.
In order to avoid slinging of dampening fluid, the metering rollers are normally limited to speeds in the range of 150 to 200 feet per minute. Thus most three and four-roller continuous contact the dampening systems manufactured today are characterized by the existence of a slip nip in the fluid path between the metering or squeeze rollers and the plate cylinder. Slip nips are characterized as nips formed by rollers travelling at significantly different surface speeds. For example, in a modern web offset press, the plate cylinder and most rollers on the press may travel at a speed of 1,500 feet per minute whereas the pair of squeeze rollers in the dampening system may be driven separately at a speed one tenth that or 150 feet per minute. Thus a slip nip must exist.
One other unique characteristic of most continuous contact three-roller and four-roller type dampening systems is that the metering or squeeze roller pair is arranged so that any fluid contained in the metering nip will drain out in a circumferential direction over one of the two roller surfaces, whenever the rollers stop turning or if excess fluid is supplied to the nip. An additional feature normally found in such dampening systems is a fountain solution circulating system, consisting basically of a pump and tank or reservoir for maintaining a constant value of fluid in the pan.
Because fountain solution is free to flow back and forth between plate cylinder and metering rollers, overfeeding of fountain solution to the plate is automatically compensated for by an increase in the back flow from plate cylinder to the metering rollers. This feature, plus the ability to vary lateral flow by skewing of one of the metering rollers, makes this class of dampeners very user friendly and capable of producing very high print quality. A drawback of this type of dampening system, however, is that the large inventory of fountain solution contained in the pan and the circulating system often rapidly becomes contaminated with ink, which results in degradation of print quality, buildup of emulsified ink on dampening system rollers, and the need to periodically clean the circulating system. Various attempts to solve these problems by installing filters to remove the offending ink particles from the dampening fluid have been largely unsuccessful.
Another drawback of existing fountain supply systems for continuous contact type dampeners is that the inventory of fountain solution must be replaced periodically with fresh solution. Due to feedback from the press, contaminants build up in the fountain solution, and these contaminants have an adverse affect on printing. The problem is especially acute when using alcohol substitutes in the fountain solution. The need to periodically replace fountain solution in the supply system oftentimes necessitates shutting down the press, which results in lost production time and lost printed product. In addition, disposal of the contaminated or waste fountain solution is becoming increasingly expensive because of ever stricter environmental regulations governing disposal of such wastes.
A continuous contact type dampener system equipped with a spray-type fluid supply system is disclosed in Marcum U.S. Pat. No. 4,481,855 entitled "Dampening Unit For Printing Press" dated Jun. 27, 1989. The purpose of the spray-type supply is to prevent pick-up of lint and debris that may collect in the pan. Thus no attempt was made to minimize either the volume in the metering nip or the amount draining away from the metering nip. Another variation of a continuous contact type dampening system is disclosed in Loudon U.S. Pat. No. 4,455,938 entitled "Dampening Apparatus for Lithographic Press" dated Jun. 26, 1984. The unique features of this design are that only two rollers are used and that both metering or squeeze rollers travel at press speed.